1. Technical Field
This disclosure is concerned with enhancing the performance of cable detection apparatus that uses harmonics of the power line frequency at which electrical power is distributed.
2. Discussion of Related Art
Cables that are buried, or inaccessible for other reasons, often need to be accurately located, e.g. to prevent them from being damaged during excavation work. Known cable detection devices typically have two or more magnetic sensors that sense the electromagnetic field emitted by single phase and three-phase power cables. Similar detection techniques can also be used for other utilities that do not carry electrical power as these conductors often pick up stray electromagnetic fields from nearby power lines.
The field emitted by a cable has a fundamental frequency generated by the alternating current at which power is distributed, generally 50 or 60 Hz, together with odd and even harmonics. The predominant energy is contained in the fundamental frequency and its odd harmonics, namely:                50, 150, 250, 350 Hz etc. for a 50 Hz power grid        60, 180, 300, 420 Hz etc. for a 60 Hz power gridFrequencies that are not harmonically related to the fundamental frequency merely contribute background noise, which decreases the accuracy of detection, and in an ideal detection system they would be completely rejected. It is also generally considered that removing the even harmonics (e.g. 100, 200 Hz etc. in a 50 Hz power system) will further increase the signal-to-noise ratio and improve detection accuracy. In a practical detector a combination of comb filters may be used to position periodic nulls in the sensed frequency spectrum. One comb filter is arranged to have notches positioned to reject the non-harmonic frequencies and another has notches arranged to remove the even harmonics.        
Users generally benefit from hearing a “real sound” output with audible power line harmonics, and for this reason the signal received from the cable is usually amplified and fed to an audio output device. This can result in major feedback problems, commonly known as “blowback”, due to electromagnetic pickup of the amplified signals from the speaker or amplifier by the field sensors. This is presently overcome by feeding the filtered signal to a mixer which shifts the frequency of the signal which is sent to the audio output device. The odd harmonics then fall within the rejection nulls of the filter system and will thus be removed if any stray output signal is picked up by the sensors.
Therefore, there is a need to improve detection accuracy, increase detection speed and reduce hardware requirements.